Anomalous Anisotropy in Superconducting Nanodiamond Films Induced by Crystallite Geometry

Superconducting thin films underpin the function of a large variety of devices. As a promising and intriguing system with tunable electronic properties, lab-grown diamond films exhibit superconductivity upon heavy boron doping. This work reveals an anomalous superconducting anisotropy in such films: In contrast to other superconducting thin films featuring a larger in-plane upper critical field, these show a larger $o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t\ensuremath{-}o\phantom{\rule{0}{0ex}}f\ensuremath{-}p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e$ upper critical field, due to quantum confinement correlated with crystallite geometry. This study provides physical insight for developing nanodiamond-based superconducting quantum devices, by exploiting grain or twin boundaries.